Dispersion strengthened metals

ABSTRACT

A dispersion-strengthened metal wherein the improvement comprises recrystallizing alloy powder prior to internal oxidation to increase the grain size of the alloy to at least about ASTM (E-112) Grain Size No. 6 to reduce the grain boundary area in the alloy powder whereby the recrystallized alloy powder provides substantially improved structural properties in the dispersion-strengthened metal product.

United States Patent 191 Nadkarni DISPERSION STRENGTHENED METALS [75]Inventor: Anil V. Nadkarni, Baltimore, Md.

[73] Assignee: SCM Corporation, New York, NY.

[22] Filed: July 30, 1973 [2l] Appl. No.: 384,028

Related U.S. Application Data [63] Continuation-impart of Ser. No.217,506, Jan. 13,

1972, Pat. No. 3,779,7l4.

[52] U.S. Cl 75/0.5 R; 75/0.5 B; 75/0.5 BA; 75/0.5 BB; 7510.5 BC; 75/206[51] Int. Cl B22f 9/00 [58] Field of Search 75/0.5 B, 0.5 BA, 0.5 R,75/0.5 BB, 0.5 BC, 206

[56] References Cited UNITED STATES PATENTS 2,539,298 l/l95l Doty et al75/O.5 BC

[451 July s, 1975 3,45l,803 6/1969 Cerulli 75/0.5 BC 3,488,183 1/1970Schreiner et al.... IS/0.5 B 3,492,113 1/1970 Shafer et a]. 75/0.5 BC3,505,059 4/1970 Cerulli 75/0.5 BC 3,552,954 l/l97l McDonald 7.5/0.5 BC

Primary Examiner-W. Stallard Attorney, Agent, or Firm-Thomas M. Schmitz[57] ABSTRACT A dispersion-strengthened metal wherein the improvementcomprises recrystallizing alloy powder prior to internal oxidation toincrease the grain size of the alloy to at least about ASTM (E-1l2)Grain Size No. 6 to reduce the grain boundary area in the alloy pow derwhereby the recrystallized alloy powder provides substantially improvedstructural properties in the dispersion-strengthened metal product.

1 Claim, 2 Drawing Figures 1 DISPERSION STRENGTHENED METALS This is acontinuation-in-part of copending application Ser. No. 2l7.5()6 filed onJan. I3. 1972. now U.S. Pat. No. 3.779.714 and said application isincorporated herein by reference.

BACKGROUND OF THE INVENTION Dispersion strengthening has been recognizedin the past as a method for increasing strength and hardness of metals.A solid solution alloy comprising a relatively noble matrix metal havingrelatively low heat or free energy of oxide formation and a solute metalhaving relatively high negative heat or free energy of oxide formationwherein the alloy is heated under oxidizing conditions to preferentiallyoxidize the solute metal. This technique is known in the art as in situinternal oxidation of the solute metal to the solute metal oxide or moresimply internal oxidation."

Dispersion-strengthened metal products. such as copper dispersionsstrengthened with aluminum oxide. have many commercial and industrialuses wherein high temperature strength properties and high electricaland/or thermal conductivities are desired or required in the finishedproduct. Such commercial uses include frictional brake parts such aslinings. facings. drums. and other machine parts for frictionapplications. Other commercial uses include electrical contact points.resistance welding electrodes. electrodes generally. electrical switchesand switch gears. transistor assemblies. wires for solderlessconnections. wires for electrical motors. and many other uses requiringgood electrical and thermal conductivities together with good strengthand hardness at elevated temperatures.

Several prior art processes for internal oxidation have been suggested.such as disclosed in the Schreiner patent. U.S. Pat No. 3.488.185; theMcDonald patent. U.S. Pat. No. 3.552.954; and the Grant patent. U.S.Pat. No. 3.179.5l5. The prior art processes require delicate controlover the partial pressure of oxygen during oxidation. or require removalof an oxidant residue which otherwise would form defects in thedispersionstrengthened metal.

Copending application Ser. No. 217.506 provides an improvedalloy-oxidant mixture by providing for assimilation of the oxidantresidue into the dispersion strengthened metal wherein the oxidantresidue is dispersion strengthened during thermal coalescence by a hard.refractory metal oxide provided in the oxidant. The oxidant residueformed during internal oxidation is not required to be removed from thedispersionstrengthened metal but rather is dispersion strengthened bythe hard. refractory metal oxide during coalescence to form an integralpart of the dispersionstrengthened metal stock.

It has been found that dispersion-strengthened metals produced byinternal oxidation have substantially improved properties if the alloypowder is recrystallized prior to internal oxidation in order toincrease the grain size of the alloy powder to at least ASTM Grain SizeNo. 6 as measured by ASTM Test No. E-l l2.

Accordingly. it is a primary objective of this invention to increase thegrain size and reduce the grain boundary area in the alloy powder priorto the step of internal oxidation in processes for dispersionstrengthening of metals.

A further objective and advantage of this invention is to providereduced grain boundaries to substantially eliminate the concentration ofsolute metal oxide at the grain boundaries of the alloy during internaloxidation. This provides a dispersion-strengthened metal product havingsustained resistance to preferential stress failure at the grainboundaries.

A still further objective is to provide a dispersionstrengthened metalproduct having a finer and more uniform distribution of dispersoidparticles which result in improved elevated temperature properties.

A further object of this invention is to minimize formation of inneroxide film of solute metal oxide within the alloy during internaloxidation by increasing the grain size of the alloy prior to internaloxidation which advantageously permits higher oxidation temperatures aswell as decreasing oxidation time.

A still further objective is to permit increased amounts of solute metaloxide which advantageously provide improved strength propertiesparticularly at elevated temperatures.

A still further objective is to dispersion strengthen larger alloyparticles which advantageously permits increased yield.

These and other advantages will become more apparent from the detaileddescription of the invention.

SUMMARY OF THE INVENTION Briefly. this invention provides analloy-oxidant mixtureadapted to be internally oxidized wherein the alloypowder is recrystallized at elevated temperatures prior to the step ofinternal oxidation for a time sufficient to increase the grain size ofthe alloy powder to at least about ASTM (E-l l2) Grain Size No. 6.

In the drawings:

FIG. I is a microphotograph magnified 500 times showing internallyoxidized aluminum alloy powder without prior recrystallization; and

FIG. 2 is a microphotograph magnified 500 times indicating internallyoxidized aluminum alloy powder recrystallized prior to internaloxidation in accordance with this invention to obtain a grain size No. 6as measured by ASTM Test E-l l2.

DETAILED DESCRIPTION OF THE INVENTION This invention pertains todispersion-strengthened metals produced by internal oxidation of alloypowders wherein the improvement comprises recrystallizing the alloypowder to increase the grain size of the alloy prior to the internaloxidation step. Thereafter. the recrystallized powdered alloy issuitable for intimately intermix ing with oxidant. internally oxidized.and then co alesced and formed into dispersion-strengthened metal stock.

Referring first to the drawings. FIG. I indicates a copper alloycontaining about 0.70% by weight of aluminum which was internallyoxidized but without prior recrystallization. Shown on the outerperiphery of the copper alloy I0 is a continuous internal oxide film 12which has been found to inhibit internal oxidation of the aluminum. FIG.2 similarly indicates the same alloy composition. internally oxidized.but recrystallized prior to internal oxidation to achieve a grain sizeNo. 6 as measured by ASTM Test E-l 12. No oxide film is formed on theouter periphery of the recrystallized alloy 14 as compared tononrecrystallized alloy 10 shown in FIG. 1.

Referring now to the powdered alloy. the preferred powder alloycomprises a relatively noble matrix metal having a negative free energyof oxide formation at C of up to 70 kilocalories per gram atom ofoxygen. and a solute metal having a negative free energy of oxideformation exceeding that of the relatively noble matrix metal by atleast about 60 kilocalories per gram atom of oxygen at 25C. Therelatively noble matrix metal and the solute metal are alloyed byconventional techniques such as melting them under inert or reducingconditions and thereafter comminuting the alloy by atomization or otherconventional size-reduction techniques such as grinding or ball millingto form a particulate alloy having an average particle size of less thanabout 300 microns. Suitable noble matrix metals include. for example.iron. cobalt. nickel. copper, cadmium. thallium. germanium. tin. lead,antimony. bismuth. molybdenum. tungsten. rhenium. indium. silver. gold.ruthenium. palladium. osmium. platinum. and rhodium. Suitable solutemetals include. for example. silicon. titanium. zirconium. aluminum.beryllium. thorium. chromium. magnesium. manganese. The alloycomposition comprises about 0.01% to about 2 weight percent of solutemetal with the balance being relatively noble matrix metal and. ifdesired, minor amounts of conventional additives to improve abrasionresistance. hardness. conductivity. and other selected properties.

In accordance with this invention. the comminuted alloy powder is firstrecrystallized at elevated annealing temperatures of the alloy toincrease the grain size of the alloy powder to achieve at least aboutASTM Grain Size No. 6 measured by ASTM Test No. El 12. The process ofrecrystallization ordinarily consists of annealing the alloy to producea new grain structure. Re crystallization diagrams for most metals andalloys are published. as indicated in Practical Metallurgy. by Sachs andVan Horn. particularly Chapter 5. 3rd printing. 1943. and incorporatedherein by reference. An nealing temperatures depend on the alloy to bedispersion strengthened and such temperatures are high enough toefficiently cause recrystallization but at temperatures substantiallylower than the melting point of the alloy. For a predominantly copperalloy with minor amounts of aluminum. for example. desirablerecrystallization takes place after heating for at least an hour at atemperature of about 1600F in an inert atmosphere such as argon. Muchlower recrystallization tempera tures may be utilized. such as 600F to700F. but lower temperatures necessitate increased heat times.Increasing the grain size of the alloy powder prior to internaloxidation has been found to effectively minimize the tendency for solutemetal oxide to concentrate at the powder grain boundaries duringinternal oxidation which undesirably may cause early failure understress at the grain boundaries in the final dispersionstrengthened metalproduct.

After recrystallization. the alloy powder is internally oxidized byconventional methods such as disclosed in prior art processes disclosedhereinbefore and identi lied as patents issued to Schreiner. McDonald.and Grant. and the same are included herein by reference. For example.the Schreiner patent. US. Pat. No. 1488.1 83 provides a suitable methodof internal oxidation of an alloy by controlling partial pressure ofoxygen produced by dissociation of metal oxide within a two-compartmentchamber. The McDonald patent. US. Pat. No. 3.552.954. provides internaloxidation of a copper alloy within an oxygen atmosphere to saturate thecopper with oxygen and thereafter reducing with hydrogen. The Grantpatent. US. Pat. No. 3.179.515. suggests internal oxidation by firstoxidizing a copper alloy in air to form a surface layer of Cu O followedby continued heating to diffuse oxygen into the copper matrix followedby hydrogen reduction.

A particularly preferred method of internally oxidizing is disclosed insaid copending application. Ser. No. 217.506. and provides for anintimate admixture of alloy powder with oxidant. The disclosed oxidantcomprises a pulverant. in situ heat-reducible metal oxide having anegative free energy of formation ranging up to kilocalories per gramatom of oxygen at 25C in intimate interspersion with discrete particlesof hard. refractory metal oxide. the negative free energy of formationof said hard. refractory metal oxide exceeding the negative free energyof formation of said heat' reducible metal oxide by at least 60kilocalories per gram atom of oxygen at 25C. Suitable heat-reduciblemetal oxides include. for example, oxides of iron. cobalt. nickel.copper. cadmium. thallium. germanium. tin. lead. antimony. bismuth.molybdenum. tungsten. rhenium. indium. silver. gold. ruthenium.palladium. osmium. platinum. and rhodium. Suitable hard. refractorymetal oxides include. for example. oxides of silicon. titanium.zirconium. aluminum. beryllium. thorium. chromium. magnesium. manganese.

In any particular combination of the matrix metal and the solute metalin the alloy to be internally oxidized by the preferred method. thematrix metal must be relatively noble with respect to the solute metalso that the solute metal will be preferentially oxidized. This isachieved by selecting the solute metal such that its negative freeenergy of oxide formation at 25C is at least 60 kilocalories per gramatom of oxygen greater than the negative free energy of formation of theoxide of the matrix metal at 25C. Generally. such solute metals have anegative free energy of oxide formation per gram atom of oxygen of overkilocalories and preferably over 120 kilocalories. Similarly. the metalmoiety of the heat-reducible metal oxide in the oxidant preferably isthe same metal as matrix metal present in the alloy to be internallyoxidized. although the heatreducible metal oxide moiety can bedifferent. Similarly. the hard. refractory metal oxide in the oxidantpreferably is the same as the solute metal oxide formed in the alloyduring internal oxidation of the alloy. although the refractory metaloxide in the oxidant can be different from the solute metal oxide in theinternally oxidized alloy. as more particularly set forth in saidcopending application. Ser. No. 217.506.

As indicated. the oxidant for internally oxidizing the powdered alloy bythe preferred method is a mixture of an in situ heat-reducible metaloxide and a hard. refractory metal oxide. Several methods of forming asuitable oxidant are described in said copending application Ser. No.217.506 and include. for example. decomposing an oxide'forming salt of arefractory metal on particles of heat-reducible metal oxide in themicron or sub micron range. or coprecipitation of oxide-formingcompounds from their respective salt solutions. or physical blending ofthe desired oxide components.

In preparing the alloy-oxidant mixture in accordance with the preferredmethod. at least about 0.1 weight parts of oxidant are combined withweight parts of powder alloy and desirably between about 0.1 to 20weight parts of oxidant. Preferably. about 0.1 to 10 weight parts ofoxidant are combined with about I00 weight parts of powder alloy. Theexact proportions of the oxidant relative to the alloy depends on thesolute metal of the alloy to be oxidized, and the oxygen content of theoxidant. The amount of such oxidant to be added may be determined by thestoichiometric amount of oxygen required to completely oxidize thesolute metal. in this regard. the heat-reducible metai oxide is added insufficient amounts to completely oxidize the solute metal in the alloy,whereas the amount of hard, refractory metal oxide depends upon theamount of heat-reducible metal oxide. The residue of heat-reduciblemetal oxide present after internal oxidation is dispersion strengthenedby coalescence by the hard, refractory metal oxide. Sufficient oxidantis utilized to completely oxidize the solute metal in the alloy.however. if excessive oxidant is utilized. the resulting internallyoxidized metal powder may then be reduced with hydrogen at temperaturesof about l500F for time sufficient to reduce residual oxygen.

The following illustrative examples are included to further explain theinvention and are not intended to be limiting All parts are by weightand all temperatures are in degrees Fahrenheit. unless otherwise stated.

EXAMPLE 1 Part A Preparation of the Alloy Powder Electrolytictough-pitch grade copper rods are melted in an inert refractory cruciblein an inductionheating furnace under reducing conditions at about 2300F.Metallic aluminum shavings are introduced into the molten copper in theproportion of 0.33% by weight of the resulting molten metallic mass.

The molten solution of aluminum in copper is then superheated to 2400F.atomized through an atomizing aperture in a jet of nitrogen(alternatively other inert gases or water or steam can be used as theatomizing fluid) to yield an atomized copper-aluminum alloy powder whichsubstantially all passes a lOO-mesh. U.S. Sieve indicating that theaverage particle size is less than about I49 microns.

The atomized and screened alloy powder is annealed at a temperature ofabout l600F for about an hour in an argon atmosphere to recrystallizeand yield a grain size in the recrystallized alloy powder of at leastabout ASTM Grain Size 6 according to ASTM Test E-l12. Preferably. thegrains are as large as possible to minimize grain boundary area in thepowder. The alloy powder is then ready for use in combination with theoxidant.

Part B Preparation of the Oxidant One hundred parts of commerciallyavailable cuprous oxide (Cu O) with an average particle size of about 1to 2 microns are mixed with 4.l parts of an 7 aqueous solution ofA1(NO,-;);, 9H O to form a slurry of cuprous oxide in aluminum nitratesolution. The solution of aluminum nitrate is slurried with cuprousoxide particles. and the stirring is continued with mild heating at 200Funtil the water has evaporated and the mixture is almost dry. Themixture is then heated at a temperature of about SOO F for hour todecompose the aluminum nitrate into aluminum oxide. The resultingagglomerate is then ground to form fine oxidant powder which passes a325-mesh sieve. The resulting oxidant powder comprises 77.4371 Cu- O.22.01% CuO, and 0.56% M 0 by weight.

Part C Preparation of the Internally Oxidizable Alloy Powder-oxidantMixture The alloy powder of Part A is thoroughly mixed with the oxidantpowder of Part B in the proportion of 2.12 parts of oxidant to I00 partsof alloy powder. The mixing is accomplished in a ball-mill. although aconventional V-cone blending device can alternatively be used.

Part D Internal Oxidation of the Alloy Powder The alloy powder-oxidantmixture of Part C is then charged to an internal oxidation vessel whichis then sealed. The oxidation vessel is copper or copper-lined steel toavoid contamination of the alloy powder-oxide mixture during oxidation.

The alloy powder-oxidant mixture is then brought to a temperature ofabout l750F and maintained at this temperature for about 30 minutes toeffectuate internal oxidation of the alloy powder. Alternatively, theinternal oxidation can be carried out on a continuous basis using acontinuous belt furnace maintained under an inert atmosphere.

At the end of the 30-minute internal oxidation period. substantially allof the aluminum in the alloy powder has been oxidized to M 0 andsubstantially all of the cuprous oxide in the oxidant has been reducedto metallic copper. The particles of internally oxidized alloy comprise99.37% by weight of copper plus minor amounts of impurities and 0.63% byweight of M 0 The oxidant residue comprises 99.3771 copper particles and0.63% A1 0 particles. The overall internally oxidized metal powdercomposition comprises 98.2 I'/( internally oxidized alloy powder and1.79% oxidant residue.

Part E Reduction of the Internally Oxidized Metal Powder The internallyoxidized metal powder of Part D is then placed in a reducing atmosphereof hydrogen at a temperature of about l500F for one hour to reduce anyresidual copper oxide.

Part F Thermal Coalescence or Consolidation of the internally OxidizedMetal Powder The internally oxidized and reduced metal powder of Part Eis then changed under an inert argon atmosphere to a thin-walled coppercan having a diameter of about 7 inches and equipped with a feed tube.The can and its contents are heated to about l700F and the feed tubesealed. Alternatively instead of using the inert gas atmosphere. thefeed tube is attached to a vacuum pump; and the can is evacuated whilethe temperature of the can is brought to l700F to remove any occludedgas from the powder. After evacuation at a pressure of l X l0 mm of Hgfor minutes at 1700F. the feed tube is sealed and disconnected from thevacuum pump.

The sealed can is then placed in a ram-type extrusion press and isextruded to form extrudate in the shape of cylindrical bar stock havinga diameter of about L25 inches. This corresponds to an extrusion ratioof about 3l:l (i.e., the ratio of the cross-sectional area of the can tothe ratio of the cross-sectional area of the extrudate).

The bar stock comprises about 99.37% copper having dispersed throughout0.637: (or about I 5% by volume) of M 0 particles and has a density ofabout 99.3% of the theoretical density. The bar stock has an electricalconductivity of 88% lACS". a tensile strength of about 72.000 psi, anelongation of 19% using ASTM Test E-8 (for a test specimen 0. lo inchdiameter and 0.65 inch gage length) and a Rockwell hardness of about 75units on the 8 scale. All property measurements reported in the exampleare conducted at room ized powder are each blended with stoichiometricamounts of oxidant consisting of an intimate mixture of submicron Cu Oand N and internally oxidized, as indicated in Example l. at 1750F inargon for about temperature. The bar stock is substantially uniform and5 minutes. Both the non-treated and the recrystallized does not P055655Compositional l mixtures are then reduced at l500F for one hour in mallyresult when the spent oxidant is present in the hydrogen to remove anyexcess Oxygen. The reduced dlspel'slon'slrengtheged Workplecepowdermixtures are then cold compacted in a rectan- *lntcrnational Annealedoppcr Standard A copper wire I mctcr long weighing 1 gram. having aresistance 01015328 ohms. at 20C has X0 glfldr Cm X 1 cm i a Compactingpressure 01:40 a conductivity of 100% lACS (Kirk-Othmcr: Encyclopediaol'Chcmi- (SI [0 yield a compact with l cm square cross section,Tiglgrolgg iagcycond Edition. Volume Vl. lnterscicncc Publishers. Thcompact i h f d to 993% d i d h The bar stock is suitable for use as is.or it can be cold Cold forged to give about r educuon in area vick'worked by swaging, forging. rolling. wire drawing. cold i S f ii are hif i iglz ff extrusion or cold drawing to form workpieces having H t enmoedwre a I l f treatment at [500 F for an hour in argon. The Vicker sparticular tensile strengths according to conventional Cold workingtechniques (DPH) Diamond Pyramid Hardnesses are measured in insumce whenthe bar Stock is reduced to 50% kilograms per mm at a 15 gram load andthe results in cross-sectional area by coldswaging. the tensile thereofare mdlcmed Table 1 belowstrength is 80.000 psi, the elongation is 1371,and Rock- 2 Tflbl 1 Shows that higher mhel'em hardnesses well B hardnessis 84 units and conductivity is 86% obtained when the atomized alloypowder is recrystallACS. lized rior to internal oxidation concurrentwith an imp This swaged material with a Rockwell B hardness of provedresistance to softening upon further heating to 84 units and prepared bythe procedure of Example I l d temperatures.

TABLE 1 Vickers Hardness Grain Size Vicker's Hardness DPH (annealed lhr. at Mesh (average DPH (as forged) 1S00F) kglmm at Sample No. FractionProcessing diameter) kg/mm at 15g. load 15 g. load 2(a) -80+325 withgrain 0.045 mm l49 kg/mm I39 kg/mm growth 2(b) 80+325 without 0.00397 mm122 kg/mm 103 kglmm grain growth 2(c) --325 with grain 0.045 mm 153ltg/mm l53 ltg/mm growth 2(d) 325 without 0.00397 mm [50 kg/mm l32 kg/mmgrain growth is annealed along with a commercial copper-chromium EXAMPLE3 F 1 I t alloy (0.9/r Cr) at various temperatures for one hour 40 Analloy of Copper huvmg 0.70% aluminum ls in argon. Improved hardnessvalues are obtained by anpared in a manner set forth in Emm e l an A)one nealing for one hour at the various temperatures rangf p ing from100F to 1500F. In another experiment. these Sample 0 duo) pc'wdfirprocebbed m a same two materials are annealed together at l000F innersctlorth i Example 1 mcludmg the step of recrys' argon Samples areremoved from the annealing tallization to increase the grain size priorto internal oxnace at various time intervals. cooled to room temperaf Aseconfl Sample (b) Processed slmllarly but ture. and tested forhardness. The test results show su- Wllh the exception that [ha gramgrowth p perior resistance to softening upon heating of the diseluded.Both powd and are d, P persion-strengthened workpiece of this invention.ished. and etched. The non-recrystallized (no grain growth) powder (b)shows a continuous internal oxide EXAMPLE 2 film after internaloxidation. whereas the recrystallized (grain growth) powder (a) does notindicate an oxide A copper alloy similar to Example I and containing mvi k Diamond pyramid Harm-3SS (DPH) weight Percent of aluminumnllrOgellFmm'lZed 5g taken at l5 g. load on the internally oxidizedpowders l P' 9) powder- The alloy p'owder dmfied (a) and (b) afterinternal oxidation in a manner set g a?- f 2 fl s g 9a; forth in Example1 indicate the following hardnesses: and a -3... mes raction. ac ract onis rea e wi Recrystallized powder (a): 89 kg/mm2 a grain growth step andcompared with a fraction that u I Non-recrystallized powder (b):94kg/mminside the did not undergo grain growth. ln the grain growthtrac- & I] n 161 k 2 d t h tion. the atomized powder is subjected torecrystallim 1 :1} e g mm outs e or t e zation treatment at l800F forone hour under argon Oxlde I atmosphere prior to internal oxidation.Prior to recrysh Powders and are t n Canned in a 1.25 tallization. theaverage grain diameter is 0.00397 millimch diameter and 2 Inch longcopper containers and meters. whereas after recrystallization theaverage extruded at l700F into 0.25 inch diameter rods. Rockgraindiameter is 0.045 millimeters. Both the nontreated atomized powder andthe recrystallized atomwell hardness. electrical conductivity andultimate tensile strength are determined and are set forth in Table 2.

TABLE 2 Electrical Sample Rockwell Conductivity Tensile No. ProcessingHardness IAC S Strength 3( a) with grain 90 79% 85.000 psi growth 3th)without 83 75% 80,000 psi grain growth EXAMPLE 4 Two fractions of -lmesh copper alloy powder containing 0.70% aluminum are surface oxidizedat 450C for about V2 hour to pick up sufficient oxygen for completeoxidation of the 0.70% Al in the alloy powder. One fraction (a) ispreviously recrystallized for increasing the grain size in the mannerset forth in Example 2. but the other fraction (b) is not subjected to agrain growth step. Both fractions (a) and (b) are internally oxidized atl750F for hour and then reduced to remove any excess oxygen. Eachfraction is individually canned within a L inch diameter and 2 inch longcopper container. preheated. and extruded at about I700F into 0.25 inchdiameter rods. Rockwell hardness. electrical conductivity. and ultimatetensile strength on these rods are shown in Table 3.

In accordance with the procedures set forth in Example l. a nickel alloycontaining 0.45% aluminum by weight is nitrogen atomized to produce analloy powder. The alloy powder is divided into two fractions. namely,one fraction recrystallized in a grain growth step. and the otherfraction did not undergo grain growth. In the grain growth fraction. theatomized nickel alloy powder is subjected to recrystallization treatmentat about l800F for one hour under argon atmosphere to achieve a grainsize No. 6. Both the recrystallized fraction and the non-treatedfraction of the nickel alloy powder is then mixed with L89 weight partsof pulverant oxidant comprising l.87 parts of nickel dioxide and 0.02parts of aluminum oxide per I00 weight parts of powder alloy. The nickelalloy and oxidant mixtures are then internally oxidized as indicated inExample I at l750F in argon for about 3 hours. Both fractions are thenreduced with hydrogen at I500F for about one hour to remove any excessoxygen. The reduced powder mixtures are then cold compacted and hotforged in the manner set forth in Example 2 and tested. Therecrystallized alloy fraction undergoing grain growth to achieve a grainsize number of at least 6 indicates substantially improved Vickershardness. Rockwell hardness. electrical conductivity. and tensilestrength when compared to the other nonrecrystallized alloy fraction.

EXAMPLE 6 A silver alloy containing 99.04% silver and 0.48% aluminum isnitrogen atomized in a manner similar to Example I to produce an alloypowder. The alloy powder is then separated into two fractions whereinone fraction is recrystallized with a grain growth step whereas thesecond fraction is not treated to undergo grain growth. In the graingrowth fraction. the atomized alloy powder is subjected torecrystallization treatment at about l500F for about one hour underargon atmosphere to achieve a grain size of No. 6. Both therecrystallization and the non-treated fractions are combined with 6.35parts of pulverant oxidant comprising 6.24 parts of silver oxide and 0.11 parts of aluminum oxide. Both fractions are then internally oxidizedat 1200F in argon for about I hour in the manner indicated in ExampleEach fraction is reduced. compacted. and hot forged as indicated inExample 2. The recrystallized fraction exhibits substantially improvedVickers hardness. Rockwell hardness. electrical conductivity. andtensile strength when compared to the non-treated fraction.

The foregoing examples indicate that recrystallizing and increasing thegrain size of the alloy powder prior to internal oxidation substantiallyimproves the physical properties of dispersion strengthened metalproducts. The examples are not intended to be limiting to the scope ofthis invention as defined in the following claims.

I claim:

1. An improved powdered alloy suitable for dispersion strengthening byinternal oxidation. comprising:

an alloy comprising a relatively noble matrix metal having a negativefree energy of oxide formation at 25C of up to kilocalories per gramatom of oxygen and a solute metal having a negative free energy of oxideformation exceeding the free energy of oxide formation of said noblematrix metal by at least about 60 kilocalories per gram atom of oxygenat 25C; and

said powdered alloy being recrystallized and having 21 Grain Size of atleast Number 6 as measured by ASTM E-l l2.

1. AN IMPROVED POWDERD ALLOY SUITABLE FOR DISPERSION STRENGTHENING BYINTERNAL OXIDATION, COMPRISING: AN ALLOY COMPRISING A RELATIVELY NOBLEMATRIX METAL HAVING A NEGATIVE FREE ENERGY OF OXIDE FORMATION AT 25*C OFUP TO 70 KILOCALORIES PER GRAM ATOM OF OXYGEN AND A SOLUTE METAL HAVINGA NEGATIVE FREE ENERGY OF OXIDE FORMATION EXCEEDING THE FREE ENERGY OFOXIDE FORMATION NOBLE MATRIX METAL BY AT LEAST ABOUT 60 KILOCALORIES PERGRAM ATOM OF OXYGEN AT 25*C, AND SAID POWDERED ALLOY BEINGRECRYSTALLIZED AND HAVING A GRAIN SIZE OF AT LEAST BUMBER 6 AS MEASUREDBY ASTME - 112.